Evaluating the Traffic Capacity of the Zion–Mt. Carmel Tunnel in Zion National Park

Constructed in 1930 and recently designated as a National Historic Civil Engineering Landmark, the 1.1 mile long Zion–Mt. Carmel Tunnel has served Zion National Park in Utah well for several decades. With the passage of time, however, vehicles have become larger and this has necessitated the use of one-way operation to allow large vehicles to pass through the narrow tunnel. In recent years the number of visits to National Parks in the Colorado Plateau region has greatly increased. For example, visits to Zion National Park increased by 69% from 2010 to 2017. Accompanying the increase in visitor numbers has been an increase in traffic volume. As traffic volume has grown, two questions have become more obvious: What is the highway capacity of the Zion–Mt. Carmel Tunnel? And, how soon will the tunnel reach capacity? This paper covers a unique traffic engineering/highway capacity problem and describes: (a) the current method of operating the tunnel (both two-way and one-way operation); (b) data collection and analysis; (c) how one-way operation degrades tunnel capacity; (d) calculation of tunnel capacity and waiting times; (e) how soon the tunnel will reach capacity; and (f) alternatives for addressing the capacity problem.